Habits and behavior of the corn-field ant, Lasius niger americanus

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UNIVERSITY OF ILLINOIS

Agricultural Experiment Station

BULLETIN No. 131

HABITS AND BEHAVIOR OF THE

CORN-FIELD ANT, LASIUS

NIGER AMERICANUS

BY STEPHEN A. FORBES
STATE ENTOMOLOGIST

URBANA, ILLINOIS, DECEMBER, 1908

CONTENTS OF BULLETIN No. 131

PAGE

Introductory f. 31

Contents oi the Nests 31

Beginning oi a New Colony 32

Size oi Colonies . : 34

Intercolonial Hostilities 35

Area occupied by a Single Colony 37

Relations to other Species oi Ants 38

Behavior within the Nest 38

Adaptation oi Behavior to changing Conditions 40

An Injury to Corn by Ants 41

Effect oi a Change oi Crop 42

30

HABITS AND BEHAVIOR OF THE CORN-
FIELD ANT, LASIUS NIGER
AMERICANUS

BY STEPHEN A. FORBES, STATE ENTOMOLOGIST

The little brown ant notorious for its injuries to corn, and called
by us, consequently, the corn-field ant, is not by any means limited to
corn fields, but is abundant in all cultivated lands, in pastures and
meadows, in dense forests, along hard pathways, and in the sandy soil
of dry sunny roads. One sometimes finds it nesting in rotten wood
or under bark, logs, or stones, and even opening up its underground
burrows to the surface between the bricks of sidewalks and pavements.
It is distributed "over the whole of North America, except the extreme
southern and southwestern portions, from the tree line on the highest
mountains to the sands of the shore."* Wheeler says that it is the
most abundant of our ants, and hence of all our insects.

Its homes and habits have been chiefly studied in corn fields, and
there it forms rather extensive settlements, mainly centered in the hills
of corn, several adjacent hills so occupied by it being connected by un-
derground channels by way of which members of the same family may
pass from hill to hill. This is partly, no doubt, because in corn fields it
is usually in possession of plant-lice which live on the roots of corn and
which contribute to the support of the ants the fluid surplus of their
own food, but partly also because in the corn hills it is undisturbed by
the cultivator, which is likely to tear up its nests if they are established
between the rows.

CONTENTS OF THE NESTS

In the burrows of this ant one may find a rather mixed and varied
population, consisting of the eggs, larvae, pupae, males, females, and
workers of the ants -themselves, together with the various species of
root-lice harbored by them and certain kinds of mites which share its
underground habitations on terms of mutual toleration if not of active
friendship. In clover fields it is very likely to have in its nests many
mealy-bugs (Pseudococcus trifolii Forbes) of a species which infests
the roots of the clover plant, and these it treats as it does the root-lice
of the corn plant, seizing them and carrying them away when its nest
is disturbed, just as it hurries out of sight its own maggotlike larvae,
its egglike pupae, and its minute, spherical white eggs.

The contents of the nest are not precisely the same at all times of
the year. In winter, for example, one finds in it no males or pupae of
the ants, as a rule, but only workers and larvae, companion mites, and
the eggs of root-lice. In some of the large winter nests one -or more
wingless queens or mother ants may be found, altho we have not been

*An Annotated List of the Ants of New Jersey, by Wm. Morton Wheeler. Bull. Am.
Mus. Nat. Hist., Vol. I., p. 393.

31

32 BULLETIN No. 131 [December,

able to satisfy ourselves that this is true of all, or even of most, of the
winter communities of this species. A careful search and exploration
of all the tunnels and chambers of large nests have often failed to bring
to light a single queen. -Sometimes, however, two or more queens may
be seen living contentedly in the same worker family, performing their
proper function of laying eggs for the increase of the colony. Besides
these large composite and evidently well-established communities, one
may often find single females in the ground, sometimes wholly alone,
and sometimes with a few of their own eggs, a few larvae, and a small
number of workers which are mainly undersized, but with no root-lice
in possession and no companion mites. Late in fall these scattered
solitary females may have nothing with them in their pocketlike under-
ground cells except a small cluster of their own eggs. These minor
groups with a single female in charge, are the beginnings of a new
family, and do not often reach more than a score or so of individuals
by the end of the first year. The larger compound groups are older
families, how old in any given case we have no present means of
knowing.

BEGINNING OF A NEW COLONY

Beginning now with a single female, which came out from an
established colony as a winged ant but later broke off her own wings,
burrowed in the earth, and began to lay eggs for another generation,
we will follow the history of the new enterprise thru the first year,
so far as our notes and observations enable us to go. Females and
males hatching from pupae as winged ants in the underground nests
from June to October, swarm out of their burrows as if by common
consent in August or September. Such an occurrence was noticed by
Mr. H. Carman, at Urbana, at 5 p. m. September 14, 1885. Males
and females came rapidly up from their burrows under ground and,
climbing the nearest blades of grass, took flight one by one, in various
directions, the workers in the meantime running rapidly about in a
state of great excitement. The males perish before winter, and the
scattered females go into the ground, each making for herself an oval
or spherical cavity, the beginning of a new family home. Some of
these buried females begin to lay eggs in summer and fall — August 15
to November 10, as we have seen them — but others live there alone
until spring, depositing their first eggs, according to our observations,
from the first to the middle of May, and continuing to lay additional
eggs, a few at a time, until September. The minute, maggotlike, foot-
less, and helpless larvae begin to hatch from these eggs in June, and
this hatching process may continue until October. The first larvae to
appear are fed by the female from the contents of her own stomach,
which, as she is said to take no food during this period, are believed to
come from the nutriment stored up in her own body.* We have found

*Charles Janet has lately shown that the muscles of the wings of the female, used only
for a few hours during her whole life, break down within her body into a food supply avail-
able for the production of her eggs and the nourishment of her young. Anatomic du Corse-
let et Histolyse des Muscles Vibrateurs, apres le Vol Nuptial chez la Reine de la Fourmi
Lnsius rtiger. Limoges, 1908.

1908] THE CORN-FIKLD ANT 33

the oldest larvae full grown and beginning to pupate from the 12th to
the 16th of June, and pupation continues, of course, thruout the
season, as larvae from the later eggs successively get their growth.
The first workers to emerge from the pupae in these small colonies
come out early in July — from the 7th to the llth of that month,
according to our experience — and the last emerge in October, or pos-
sibly in November.

From solitary queens brought in from the field April 26 to May 3,
1906, and kept in the insectary under natural conditions, the first eggs
were obtained May 8, 9, 10, and 15, and the first larvae from these eggs
June 4. The length of the egg stage in the various lots deposited by
these females varied from twenty-two to twenty-eight days. The larvae
began to pupate about the middle of June, the larval period being, in
four cases, sixteen, seventeen, nineteen, and twenty-three days. The
first adult appeared in this cage July 7, and others emerged at intervals
thruout the remainder of the year, the pupal stage averaging about
eighteen days. Judging by these data, the time from the deposit of
the egg to the appearance of the adult is approximately two months.
In several cases where second egg-masses were laid on dates definitely
ascertained, these dates were found to coincide closely with the time
of pupation of larvae from the previous lot of eggs laid by the same
-female.

The notes of my field observers make no definite statements as to
the numbers thus produced during the first season of the queen's inde-
pendent life, but three families reared under observation in our insec-
tary, in 1906, gave respectively, 8, 9, and 19 workers as the final product
of the season's operations. These numbers are perhaps too small for
the average in the field, altho larger than those of families reared dur-
ing the first year by the carpenter-ant, as reported by Pricer in 1907.*
Forty-one first-year colonies of this latter species contained from 2 to
27 workers, with an average of 10; and 19 colonies of a related ant
(Camponotus ferrugineus") contained from 2 to 19 workers, with an
average of only 6.

The process of growth and multiplication are interrupted by
winter, during which the ants hibernate in a dormant state in whatever
stage they happen to have reached, resuming their activities in spring
at the point where cold weather arrested them. The workers open up
the nests to the surface, usually in late March or in April, the evidence
of this beginning of their seasonal activities being the appearance of
circular heaps of minute pellets of earth around the mouths of their
burrows. The young larvae grow little, if at all, as long as the weather
is cool, but, fed and cared for by the workers, increase rapidly in size
as soon as the weather becomes warm, the oldest of them reaching the
pupal state as early as May. Whether males and females appear in the
family colony during this second summer is not definitely known, but
it is rendered very doubtful by the small size of the family during the

•The Life History of the Carpenter-ant, by John Lessen Pricer. Biological Bulletin, Feb-
ruary, 1908, pp. 177-218.

34

BULLETIN No. 131

[December,

first spring and by Pricer's observations concerning the carpenter-ant,
from which he concludes that winged males and females do not appear
in the colony until it is more than two years old. Some support for
this conclusion as applying to our corn-field ant will presently be given.
We can not follow the history of the colony farther at this time, lack-
ing detailed knowledge of the rate of its increase and the form of its
organization. We only know that family groups enlarge to contain
several hundred workers, and larvae sometimes by the thousand, with
pupae, males, and queens in much smaller numbers.

SIZE OF COLONIES

The following table shows the number of ants of all stages con-
tained in 20 nests dug out in an oats field by Mr. Kelly August 28,
1906. The fact that the larger numbers are all multiples of 10 or 5
shows that these counts are not to be regarded as exact.

ANTS IN TWENTY NESTS, IN OATS FIELD, AUGUST 28, 1906

Workers

-
>

ft!
a

a

da

• "rt

Young

females

H
U

a

1

41

60

16

0

' 0

1

0

118

75

50

16

0

0

1

0

142

160

400

30

0

0

590

183

435

15

5

17

655

190

530

60

26

4

810

220

430

75

21

3

1

7

757

260

650

210

11

27

1158

280

340

120

12

20

772

330

150

240

6

24

750

340

150

60

0

5

555

360

750 30

13

33

1186

380

420 35

A

19

856

410

700

120

14

21

1265

475 235

115

0

16

841

540

150

250

6

29

975

585

280

125

0

16

•

1006

680

240

125

31

12

1088

740

310 180

19

15

1264

sea

200

160

16

34

1270

900

450

25

21

38

1434

It will be noticed, on examining this table — the figures for which
are arranged in the order of the number of workers in each nest — that
the first three colonies are of a different class from the remainder,
marked by the absence of winged ants of either sex, and by relatively
small numbers of worker larvae and worker pupae. These numbers are
apparently too large, however, for a first-year colony, and are probably
the result of two years' multiplication from the sole queen founder.
Possibly the third of the series, with its 160 workers, 400 larvae, and
30 pupae, is a third-year family. The small number of males and

1908] THE CORN-FIELD ANT 35

females in the remaining families as compared with the workers, larvae,
and pupae, can not be explained without a better knowledge of the
economy of this species. It will be noticed that the fully developed
nests, that is, those containing ants in the various stages due at this
season of the year, vary in number from 655 to 1434, with an average
for the 17 larger nests of 979.

It is probable that families reach their largest size in grass-lands,
where they may multiply without disturbance year after year; and
consistently with this supposition Mr. Kelly found in following the
plow in a field of grass in April, 1906, some of the largest nests of
our record. In one nest were nearly 6000 larvae and 300 workers,
with no eggs, no pupae, and no queen. Thirty-nine other nests in this
field were seemingly as large as this.

INTERCOLONIAL HOSTILITIES

The well-known but remarkable hostility of ants of one colony to
those of another of their own species is well illustrated by many of the
observations of my field and laboratory assistants.

For example, the first of April, 1906, a strange queen introduced
to a group of some forty workers of the corn-field ant confined in a
formicary, was ruthlessly attacked and killed by them as an intruder.
Precisely the same observation was made in another case April 27.

August 15, 1905, a wandering lone queen placed in a formicary
inclosure with five males from another locality killed two of them
the same day. During the following night two stray worker ants of
her species entered the formicary thru a crevice and attacked the
queen, one of them seizing her foot with its mandibles and holding
fast for an hour and a half, when, as it could not be compelled to
release the queen, it was killed to save her.

August t6, 1905, a strange male was first placed among twenty
workers in a formicary, but being instantly attacked it was transferred
to another containing only a lone queen — one who having rid herself
of her wings was presumably already fertilized. The male being dead
in the morning, the presumption is strong that the queen killed if. In
another case, occurring August 15, four males placed in the nest with
a strange queen were immediately attacked by her, and two of them
were presently killed. On the other hand, a queen ant still bearing her
wings, placed alone with a strange male August 21, made friends
with him immediately. The two continued to live in harmony, the
queen sometimes caressing the male and even apparently feeding him,
until the 26th, when she began to try to pull off her wings and refused
to have anything further to do with the male. September 6, altho still
bearing her wings, having failed in her efforts to remove them, she
began to lay -eggs. This seems a possible case of fertilization of the
queen within the formicary, but direct observations to that effect are
wanting.

August 16, a male from one formicary put in with the workers of
another was immediately pounced upon by them, and would evidently
have been killed if he had not been promptly rescued.

36 BULLETIN No. 131 [December,

August 15, worker ants taken from two different nests in the field
and placed in the same formicary began at once 'to fight. They kept
up their contest all night, and several of them were dead in the morn-
ing. By noon of the following day, however, fighting had ceased
and the two groups had separated, occupying different parts of the'
enclosure. The larger group had taken possession of a corn plant
which had been introduced for the use of these ants, and the other
colony, which established itself near the edge of the formicary, seemed
timid and cowed, apparently fearing even to feed upon the syrup
offered them. The dominant group mined actively in the earth about
the roots of the corn, and appropriated the root-lice introduced into
the cage, evidently feeling themselves masters of their domain.

April 24, 1906, a colony of 100 worker ants with 200 larvae of
various sizes, and eggs of the corn root-aphis, were brought in from
the field and established in a two-celled glass formicary. After they
had become thoroly at home, nine worker ants and a few ant larvae —
strangers to the original colony — a bunch of aphis eggs, and some
young root-aphids, were introduced into this nest, the original family
being in the orange or dark cell and the newcomers in the other cell
of this cage. As soon as the presence of the strangers was detected
they were attacked by the old colony, which dragged the adults about,
killing them one by one, ate up their larvae, ate or pulled to pieces the
root-lice introduced with them, and crushed the aphis eggs. The root-
lice and eggs brought in from their own nest they were in the mean-
time caring for as usual.

August 7, some eggs of the corn-field ant brought in from the
field and placed with a colony of workers well established in an artifi-
cial formicary, were presently found by one of the workers and delib-
erately crushed, one by one.

These hostilities of ants to strangers may extend even to those
of their own sisterhood who have been away from home too long.
For example, a young worker separated from its formicary mates
August 21, kept alone, and returned to them twenty-one days later,
was immediately attacked by them and quickly killed.

The utility to the ants of this inhospitable savagery — this spirit of
ferocious clannishness — is doubtless to be found in their social organ-
ization, and in the necessity under which they live of keeping always
an active, compact, deeply interested group of workers completely
devoted to the care and nurture of their helpless young. No young
of any animal can be more utterly dependent than those of these ants,
and their feeding and protection must be the constant, consuming care
of the whole family group.

The workers are, on the other hand, among the most active and
enterprising wanderers and foragers among insects, traveling far and
wide on foot to distances which make it necessary for them to retrace
their steps carefully, whether by the aid of the sense of sight or by
antennal senses supposed to resemble what in us is the sense of smell.
The homing instinct and the nursing instinct are in them so strong as

1908] THE CORN-FIELD ANT 37

practically to rule their lives. They go abroad in search of food for
the family, and to the family they must return, however far they may
travel and into whatever difficulties and adventures they may fall by
the way.

These facts make it necessary that there should be in the economy
of ant society some self-acting, self-regulating apparatus for keeping
the family groups wholly separate and distinct, for making sure that
every foraging worker shall find its way back to its own companions
— that it shall neither be liable to lose its way completely nor likely to
attach itself to an alien family. From this point of view it is greatly
to the general interest that it should find a welcome and a permanent
home only among its immediate kindred and habitual companions, and
nothing could make this condition more secure than a universally
hostile reception to the wanderer in every other family group. Clan-
nishness in ants has thus the same justification that it has among
savage men. It is a means of maintaining the necessary concentration
of the group for the care of the young, and hence for the preservation
of the race.

AREA OCCUPIED BY A SINGLE COLONY

Practical advantage was taken by us of this intolerance of ants
towards those of other parentage to ascertain the limits in the corn-
field of a single colony or family group. Assuming that specimens
from adjacent hills of corn are members of the same family if they
affiliate peaceably, but that they belong to different families if they
fight, I instructed my field assistant to bring the inhabitants of adja-
cent hills into contact with each other in artificial nests until all those
affiliating with the original group used as a test were distinguished
from those about them who refused peaceable affiliation. By a care-
ful application of this method at Elliott, May 23, 1906, two family
areas were thus marked out.

The group of hills included in the first experiment occupied an
area 10 hills in length by 7 in width, and 20 of these 70 hills were in-
fested by ants. Selecting the inhabitants of a hill from the center of
this tract for the purposes of this test, ants from the other infested
hills were placed with them successively. By this means it was found
that the inhabitants of ten of these hills were members of one com-
munity, harmonizing with each other perfectly when commingled, but
that those of the other nine were strangers to them, since the repre-
sentatives of these groups instantly fought when placed together. The
ten nests thus identified as related were distributed over an irregular
area about 30 feet long by 14 feet in greatest width, and were proba-
bly all connected by a network of underground channels. Hills inhab-
ited by hostile ants were in many cases immediately next to others
occupied by members of this family group.

Trie second experiment covered a plot of 130 hills, 32 of which
were occupied by the corn-field ant. The occupants of 12 of these
hills affiliated peaceably, while the remaining 20 were rejected by the

38 BULLETIN No. 131 [December,

original colony used in making the test. The family settlement thus
outlined, also irregular in shape, was about 30 feet long by 10 feet in
greatest breadth. There was rio intermingling of hostile groups in
either of these family areas, all the ants occupying hills within the
boundaries of each being on friendly terms.

RELATIONS TO OTHER SPECIES OF ANTS

Notwithstanding the almost invariable intolerance exhibited by
different families of the corn-field ant towards each other, they some-
times live on perfectly friendly terms with ants of other species, both
kinds mingling harmoniously in the same galleries among the roots of
the same rrlls of corn. One, of the small red house-ants (Solenopsis
molesta) which often infests kitchens and pantries to the annoyance
of the housekeeper, is frequently found at home in the nests of our
corn-field species. A mixed settlement of these two ants found in a
field near Urbana August 31, 1905, and brought to the insectary and
established in a jar of earth, collected their larvae and pupae into sep-
arate lots without contention, and continued for several days to feed
peaceably together from the same food supply. August 29 another
species common in corn fields, Formica schaufussi, was found by Mr.
Kelly inhabiting the nests of Lasius niger americanus, the larvae and
pupae of both being, as he says, mingled in the same heap. Trans-
ferred to a Mason fruit- jar and brought to the insectary, they lived
together for two days in a Lubbock nest without fighting; but after-
wards hostilities broke out and all the Formicas were killed save two
which remained in hiding, and their pupae were thrown by their con-
querors into the ditch surrounding the nest.

BEHAVIOR WITHIN THE NEST

From a colony of ants obtained August 15 and established in a
Fielde nest, one of seven queens was removed and placed by herself
August 16 in a glass Petrie dish with moist earth, for special observa-
tion. She had broken off her wings the previous day, and was hence
presumably fertilized, altho she apparently had not left her native
nest. She began at once working in the earth as if to make a burrow.
This is the queen which killed a male placed with her, as described on
another page. She worked restlessly in the earth for several days,
helping herself to the sugar syrup offered her, and deposited six eggs
August 25. It is commonly supposed that a fertile queen does not
feed after leaving the family nest until she has reared workers capable
of supplying her with food. Possibly her nest was kept too dry and
the syrup was taken as drink rather than as food.

Another queen brought in from the field August 15, pulled off
three of her wings within the next two days, but did not succeed in
ridding herself of the fourth until August 25. This queen was re-
ported by Mr. Kelly to partake once or twice a day of the syrup placed
in her cell. She busied herself with piling up the dirt beside a bit of
wet sponge, sometimes undoing at one time what she had done at
another. She laid no eggs, and died September 6.

1908] THE CORN-FIELD ANT 39

The significance of males in the ordinary life of the colony is
very small, and I have but few notes on their habits and behavior.
July 31 a worker, a male, and a pupa were placed together in a Petrie
dish, the pupa almost mature. This is a situation of great responsi-
bility for the worker ants, whose aid is necessary to the successful
emergence of the adult, and the single worker upon whom these labors
fell made every effort to discharge her duties alone, the male doing
nothing in her aid. The anxious and affectionate worker divided her
attentions between the helpless pupa and this idle male, caressing and
feeding him, and when, thru a misadventure, his wings were stuck to
the cover of the cell in the moisture which collected there, doing her
best to pull him loose. When the pupa was ready to yield the adult
the end of the pupa-case was bitten off by the worker ant, and its in-
mate pulled out head first in a helpless condition, unable even to stand.
In about three hours, however, it was walking about.

When the workers were moving the family from one cell of the
formicary to another, they were often seen dragging the males along
to the new quarters, as if ants of this stupid sex were unable to get
any idea of what was going on, and must be dealt with by physical
force.

The responsibility of the workers for the successful transforma-
tion of the pupa and the appearance of the adult is well illustrated by
an observation recorded by Mr. Kelly under date of August 17. In a
glass Petrie dish in which three workers and three pupae of the corn-
field ant had been placed for observation August 2, a worker emerged
from a pupa-case, two of the workers in the cell assisting. One of
these bit off the black tip of the pupa-case and slowly pulled the young
ant out, taking three minutes for 4he operation. The head of the new-
born worker was cleaned off v and its antennas were straightened o>ut
by the mouth-parts of one of its nurses, while the other loosened and
straightened its legs. At the end of sixteen minutes from its release
it made its first attempts to walk. In this, as in other cases observed,
the workers rewarded themselves by devouring the empty pupa-case.

The constant attention which the worker ants pay to their eggs
and larvae is so well known that it scarcely requires illustration. Many
of their ministrations, it is true, seem aimless and mechanical, mere
restless movements of their charges from place to place, but the con-
stant attention given by the workers to their young is an effective safe-
guard against any ordinary injury.

The .eggs, deposited by the female, are gathered together by the
workers, kept in a pile or ball convenient of transportation in a mass,
and carried up and clown in the nest according to the weather; and as
they hatch the young are separated from the mass of eggs and kept
by themselves, usually assorted according to size as they increase in
number and differ in age. Larvae and pupae are likewise commonly
kept distinct, the various lots doubtless requiring somewhat different
care.

The workers feed their larvae almost constantly from the contents
of their own stomachs, and often mouth and hover them, a mass of

40 BULLETIN No. 131 [December,

the ants clustering around and over them in a way to conceal them
from view. When the colony is established in a two-celled nest, one
covered with clear glass and the other with orange, the workers con-
vey their young to the orange cell, the light transmitted by orange
glass being, as shown by Miss Fielde, inappreciable by ants.

Even after the workers have emerged from the pupa-case they
are still watched and cared for by their more experienced relatives,
and often fed in the nest by foragers returning from outside. This
operation was illustrated in our breeding-cages when pale young
workers remaining within the orange cell were fed by the darker,
older ones from food exposed to them in the light cell of the formicary.

The nursing instinct of the workers is so overruling that it ex-
tends to all the inmates of the nest excepting only strangers of their
own species. A common resident of the nest of these ants is a species
of mite (Macrocheles mastus Banks) often found crawling about
among the eggs and larvae, altho never detected in any depredation
upon them. It is probably a scavenger of the domicil, and is thus
possibly entitled to the protection which it receives. When a nest con-
taining these mites is disturbed, they receive the same attention as its
other inmates from the alarmed and anxious workers. In several
nests for example, plowed up April 24, in an old oats field,, mites were
commonly present, and the ants seized them in their mandibles and
carried them away as they did their own young.

ADAPTATION OF BEHAVIOR TO CHANGING CONDITIONS

The movements of ants in their outdoor habitations often show
a surprising adaptation of habit and behavior to changing conditions
in the field. It was repeatedly noticed, for example, that during warm
dry weather in early spring the aphis eggs would often be brought
near the surface while the larvae of the ants were kept at a depth of
five or six inches below. The effect of this treatment must be to
hasten the hatching of the aphids and at the same time to retard the
development of the larvae of the ants. The growth of the latter is thus
kept practically at a standstill while food is scarce in spring, but pro-
ceeds at a rapid pace after the aphids have hatched and are yielding
an abundant food supply to the colony. In periods of summer drouth
the burrows are extended downward to a depth of twelve or fifteen
inches, and eggs, larvae, and pupae are carried down into relatively
moist earth. The ants then mass up in the depths of their burrows
and are rarely seen abroad, but communicate with their root-louse
herds by means of underground passageways, sometimes several feet
in length. When soaking rains come, these hidden colonies open up
their burrows to the surface again, as the ground dries off, and resume
their more active habits as general foragers.

Under certain conditions of serious deprivation and suffering,
however, this whole system breaks down, and the ants may devour, for
their own temporary maintenance, the very objects to which they have
previously devoted their entire existence. When infested plants wither

1908] THE CORN-FIELD ANT 41

and die, and no others can be found to which the aphids may be trans-
ferred, these are themselves eaten by the workers as a food material
too valuable to waste. When suffering from a lack of sufficient animal
food, the workers in our formicaries have occasionally helped them-
selves to a living pupa of their own species, or have even devoured
the larvae in their charge, to the last one. The corn-field ant is, in fact,
essentially a carnivorous insect, and forages actively for animal food,
especially when it is without a sufficient number of root-lice. Much
of our breeding-cage work came to naught thru the mysterious death
of the workers, apparently because their liking for a watery syrup was
regarded as evidence that this was a sufficient food for them. In the
field and in the insectary we have seen ants feed at various times and
under various conditions upon cutworms, carabid larvae, white-grubs,
dead May-beetles, dead beetles of the corn root-worm, dead grasshop-
pers, root-lice, myriapods, earthworms, their own pupae and larvae, and
the pupal envelopes vacated by adults on transformation. Occasionally
they have been seen injuring corn by hollowing out the soft seed either
before or after sprouting, and in one case at least they were reported
by an insectary assistant as feeding on the fibrous roots of the plant.

AN INJURY TO CORN BY ANTS

Under certain conditions, indeed, this corn-field ant may do con-
siderable injury to corn by a direct, unaided attack. During the* cool,
wet spring of 1905, when the softened kernels lay long in the earth
without sprouting, and the young plants grew very slowly for a time,
a field of corn following oats, heavily infested by ants which had no
root-lice in their possession, was considerably damaged by the ants,
which gnawed and hollowed out the seed, thus either killing it, or so
diminishing the food reserve" that the plant made a slow and feeble
start.

This field of forty acres, lying near Champaign, had been in oats
in 1904, in corn in 1903, and in grass in 1902. The corn was said by
the tenant to have been injured by root-lice in 1903, the part worst
infested yielding not over 22 bushels to the acre, while the remainder
of the field gave a yield of 55 to 60 bushels. Owing to a report that
the young corn was being injured by ants, Dr. J. W. Folsom, of the
University department of instruction, examined it for me May 31,
digging up sixty-one hills in a way to expose the whole root system.
Forty-one of these hills were in a part of the field but moderately in-
fested with ants, and thirteen were in an adjacent part in which ants
were very abundant. In both cases the hills examined were taken
as they came, one after the other in the row. Seven hills additional
were dug up here and there, because of an especially noticeable infesta-
tion. The corn in this field was unequal in' condition, the poorer hills
being most numerous where ants were most abundant. This unthrifty
corn was four or five inches high, and of a yellowish hue, while the
better plants were six or seven inches high, and of a good green color.

In the less-infested row six hills out of forty-one were infested
by the corn-field ant, one^ of them containing also the red house-ant

42 BULLETIN No. 131 [December,

(Solenopsis debilis). In five of the six infested hills a single kernel
of corn had been more or less eaten by the ants. No observations of
injury to the roots were reported, but the amount of stunted corn in
this part of the field could scarcely be accounted for except on the sup-
position that some other injury was being done than this to the kernel
in the hill. In the worse-infested part of the field thirteen hills were
examined, eleven of which were infested, and eight of them contained
kernels which had been more or less eaten by the ants. In five of the
hills all the kernels had been thus injured, and in the others only one
kernel to the hill. The ants in these hills were all the common corn-
field species except in one instance, where only Solenopsis was found.
In the seven hills selected because of their visible infestation, all the
kernels were eaten by ants in two, and a single kernel out of two or
three in each of the others, the ants in all these hills being the common
corn-field species. Rough estimates of the number of ants to a hill
ranged from ten workers as a minimum to a maximum of a thousand.
In one case only were ant larvae present.

In all these sixty-one hills the corn root-aphis was found but four
times — twice a single winged female, once a winged female with three
large and thirty small wingless females, and once four winged females
with between forty and fifty wingless ones, nearly all of them young.
There were, in fact, more of the common grass root-louse (Schisoneura
panicola] in the field than of the corn root-aphis, some on the roots of
corn, with ants in charge, and others on smartweed and ragweed roots,
and also accompanied by ants.

From these observations we may infer that ants living in the
meadow-grass in 1902 with grass root-lice in their possession, infested
the corn the following year; that they continued in the field through
1904, when a crop of- oats was raised, carrying a small percentage of
their grass root-lice over on roots of weeds in the field ; and that when
the ground was planted to corn in 1905 they were still abundant there,
but with so few root-lice in their possession that they availed them-
selves of the softened corn kernels in the earth for food, probably
gnawing away, also, root hairs and the finer roots, as they have been'
seen to do in confinement.

EFFECT OF A CHANGE OF CROP

The effect of a change of crop — from corn to oats, for example —
on the ant population of a badly infested field, was illustrated by an
account in my Thirteenth Report of the disappearance, during the
latter part of May, of both ants and aphids from an oats field formerly
in corn. By this time the weeds in the field were practically all dead,
the oats having reached a height to overshadow and sap them. A
similar occurrence, more closely observed, is reported by Mr. Kelly in
a series of notes running from April 21 to June 22, 1906. In a forty-
acre field of oats fifty nests of the corn-field ant were located and
marked near two sides of the field, one next a field of corn and the
other separated from corn by a hedge fence and a road. That this

1908] . THE CoRN-Fn-xn ANT 43

field had been heavily infested by root-lice the preceding year was
shown by the number of aphis eggs and young aphids in possession of
the ants — the aphids on roots of smartweed and grasslike weeds in the
field. The ants themselves lived mainly under ground, at least by day,
rarely opening their burrows to the surface, until the 4th of May,
when, as the weather warmed up after a heavy rain, they made their
presence known by a deposit of pellets of earth at the surface around
the openings of their burrows. The oats at this time were about four
inches high, and smartweeds and ragweeds about half as tall. Both
the latter had many root-lice on them, but none were on the roots of
the oats, and the aphis eggs were not yet all hatched. Nests of the
ants were very abundant in the grass outside the borders of the field,
and several of these which were opened up contained corn root-aphids
on the roots of the grass. From the 8th to the 10th of May the ants
were actively running over the ground as if in search of food, but, so
far as could be seen, with meager result. The second generation of
aphids had by this time begun to appear, and many of the smartweeds
were dead at the root. The ground was very dry for the next few
days, and both grain and weeds grew very slowly, many of the oat
plants being dead. Thus matters went on, with no material change
except that the ants were found feeding freely on dead June-beetles
and other insects, until June 5, when it was noticed that the ants were
leaving the field.

"As I was returning home last evening," says Mr. Kelly, "I no-
ticed crossing the lane beside this field a colony of Lasius niger ameri-
canus, composed of ants going in both directions between the field of
oats and the corn field on the opposite side of the road. I followed
the column thru a hedge fence and about twenty feet into the oats,
where they were coming out of their nest. Some of those going out-
ward were carrying larvae, but none of those returning in the other
direction. The line of march was indirect, and about fifty-seven feet
long, ending in the grass on the opposite side of the road, where a
new nest was being formed. The colony was still moving at 6 the
following morning, evidently having been at work all night. They
had finished the transfer before 8 a. m., nothing remaining in the nest
from which they had emerged. No root-lice were in their possession,
but only their own larvae."

Finally, on the 22d of June, a thoro search was made of the site*
of all the nests marked as originally occupied by ants. Forty-nine of
the fifty were identified, the remaining one being lost. Forty-four of
these nests had been completely deserted, and only five were still in-
habited by ants. ' These were in rather open spots in the field, with an
abundance of grass about them. There were no corn root-lice on this
grass, however, but only the grass root-louse (Schisoneura panic old)
and a few Geoica squamosa. The oats at this time reached about to
the knee, and were beginning to head.

From these data, combined with those previously published, we
may infer a gradual but general migration of the ants from the old

44 BULLETIN No. 131 [December,

corn field, now in oats, to more favorable locations, — a migration which
included all the ants, of whatever age or condition, but in which their
aphid possessions were apparently sacrificed — probably eaten by their
owners as a last resort of domestic economy. Not all the aphids of
the oats field were thus lost, however, but some seem to have been
transplanted to grasses near by, and others, which acquired wings as
food began to fail, escaped by flight, Such winged aphids were re-
peatedly seen to be captured by wandering ants, who took them in
charge and placed them on the roots of suitable plants. Others, light-
ing by chance on a corn plant, made their own way down the stalk, and
thus found a lodgment on the roots, where they were doubtless after-
wards found and cared for by ants. Nevertheless, the loss of root-
lice consequent upon a change of crop, altho not complete, must have
been enormous, only now and then one of the aphis inhabitants of the
field being fortunate enough to fall upon favorable conditions; and
crop rotation remains one of the most effective means of checking the
multiplication of these destructive insects.

1908]

THE CORN- FIELD ANT

45

PLATE I.

STAGES OF THE CORN-FIELD ANT (Lasius niger americanus) : worker; larva;
winged male ; pupa ; winged female ; female with wings removed.

BF» J SP? l/a% JrP>4»L/J» F>«^HBI